A plasma display panel (hereinafter also referred to as “PDP”) is suitable for displaying a high-quality television image on a large screen. Thus, there has been an increasing need for various kinds of display devices using the plasma display panel.
The PDP (for example, 3-electrode surface discharge type PDP) comprises a front panel facing the viewer and a rear panel opposed to each other. The front panel and the rear panel are sealed along their peripheries by a sealing material. Between the front panel and the rear panel, there is formed a discharge space filled with a discharge gas (helium, neon or the like).
The front panel is generally provided with a glass substrate, display electrodes (each of which comprises a scan electrode and a sustain electrode), a dielectric layer and a protective layer. Specifically, (i) on one of principal surfaces of the glass substrate, the display electrodes are formed in a form of stripes; (ii) the dielectric layer is formed on the principal surface of the glass substrate so as to cover the display electrodes; and (iii) the protective layer is formed on the dielectric layer so as to protect the dielectric layer.
The rear panel is generally provided with a glass substrate, address electrodes, a dielectric layer, partition walls and phosphor layers (i.e. red, green and blue fluorescent layers). Specifically, (i) on one of principal surfaces of the glass substrate, the address electrodes are formed in a form of stripes; (ii) the dielectric layer is formed on the principal surface of the glass substrate so as to cover the address electrodes; (iii) a plurality of partition walls are formed on the dielectric layer at equal intervals; and (iv) the phosphor layers are formed on the dielectric layer such that each of them is located between the adjacent partition walls.
In the PDP, the display electrode and the address electrode perpendicularly intersect with each other, and such intersection portion serves as a discharge cell. A plurality of discharge cells are arranged in the form of a matrix. Three discharge cells, which have red, green and blue phosphor layers in the arranged direction of the display electrodes, serve as picture elements for color display. In operation of the PDP, ultraviolet rays are generated in the discharge cell upon applying a voltage, and thereby the phosphor layers capable of emitting different visible lights are excited. As a result, the excited phosphor layers respectively emit lights in red, green and blue colors, which will lead to an achievement of a full-color display.
Recently, miniaturization of the discharge cells has been promoted by a demand for a higher definition of the PDP. For example, it is necessary to form the partition walls at 100 μm pitch on the rear panel in order to achieve the higher definition. However, a size reduction of the discharge cells leads to a decrease in emission brightness and thus an increase in power consumption. This is caused by a decrease in an opening ratio, a decrease in light emission time per picture element attributable to an increase in picture element number, a decrease in luminous efficiency or the like. As a method for increasing emission brightness, there has been proposed a method of increasing the opening ratio by decreasing the width of partition walls of the rear panel. However, even in this method, the emission brightness is still insufficient and a further improvement is required.
There has been proposed another method wherein a dielectric constant of a dielectric body in a front panel is decreased, and thereby reducing a reactive power upon discharge so as to improve the luminous efficiency. According to a formation of a front-sided dielectric layer in current method for producing PDPs, a dielectric material which contains glass powder with a size of several μms, an organic binder and a solvent is applied onto a glass plate by a known process such as screen printing process, die coating process or the like. Subsequently, a dielectric layer is formed from the glass material by a drying step, a debindering step (300 to 400° C.) and a calcining step (500 to 600° C.). However, as for current dielectric materials, the glass powder tends to be melted at a low temperature, and thus a “material capable of decreasing a melting point of the glass (e.g. Bi)” is added thereto (see, for example, Japanese Patent Kokai Publication No. 2002-053342). Such material capable of decreasing a melting point of the glass has low purity and has a high dielectric constant of 10 or more. Although the dielectric constant can be decreased by adding other substances (e.g. alkali metal), a highly conductive metal such as silver is used as a main component in an electrode of PDP, and thus a diffusion and colloidization of the silver are promoted due to ion migration, which leads to an yellowing phenomenon in the dielectric body. The yellowing phenomenon has a great adverse influence on the optical characteristics of PDP.
In order to increase emission brightness by decreasing the dielectric constant of the dielectric layer, it is necessary to develop a new low dielectric constant material to replace current types of glass paste, and also develop a method of forming a dielectric layer using such material. As a process for forming a dielectric layer made of high-purity oxide, there has been a process in which a solid oxide is deposited on a substrate by sputtering process under vacuum atmosphere (i.e. sputtering deposition process), and also there has been another process in which a material is deposited by decomposing a raw material with plasma (i.e. chemical vapor deposition process). Although these processes can produce a dielectric layer with a high purity and a low dielectric constant, expensive vacuum facilities are required and a film-forming rate is so low as about several 100 nm per minute. In this regard, for preventing a dielectric breakdown phenomenon upon application of voltage, the required thickness of the dielectric film is usually 10 μm or more and thus the larger number of the equipments are required to increase a productivity thereof.
Alternatively, it has been proposed to melt silica with high purity. However, the melting of such silica is not practical since a high temperature of 1000° C. or higher is required.
As a process for forming a dielectric layer with low dielectric constant while ensuring productivity, there has been proposed a sol-gel process. According to this process, a metal alkoxide is hydrolyzed in a solvent to give a silicon compound and subsequently the silicon compound is subject to a condensation polymerization treatment by heating thereof to form a film which mainly consists of silicon oxide. For example, in a case where the silicon compound is a silicon hydroxide (Si(OH)4), a network of —Si—O—Si— is formed by the following condensation polymerization reaction and thereby a solid SiO2 is formed to give a dielectric layer.nSi(OH)4→nSiO2+2nH2O                (n: an integer of 1 or more)In a case where the silicon compound is a siloxane, a dielectric layer is formed by the following condensation polymerization reaction.        

According to the sol-gel process, a dielectric layer can be formed with a low production cost and a short takt time since the existing facilities are available for the application of the raw material paste. Furthermore, according to the sol-gel process, the dielectric layer can be formed at a lower temperature since no melting of the glass is required. However, a cracking phenomenon generally occurs in the dielectric layer as a result of a volume shrinkage thereof attributable to the condensation polymerization reaction (see FIGS. 7 and 8). For this reason, it is generally difficult to form a thick film of the dielectric layer (for example, it is generally difficult to form a dielectric layer with a thickness of about 100 nm).
There have been also proposed a method of preventing the cracks from occurring wherein polysiloxane material is modified from a fully inorganic material to a material with an alkyl group so that the alkyl group remains after condensation, thereby decreasing the difference in thermal expansion between the dielectric layer and the glass substrate/electrodes upon the heating of the dielectric material (see, for example, Japanese Patent Kokai Publication No. 2003-518318). In operation of the PDP, however, the residual alkyl group may be gasified and thus the gasified gas may deteriorate the phosphor layer of the rear panel, which will lead to a lower brightness of the PDP.
Under the above circumstances, the present invention has been created. Thus, an object of the present invention is to provide a method for producing a PDP in which a deterioration of the brightness is prevented when used the method being capable of effectively preventing or reducing a cracking phenomenon which may occur upon the formation of the dielectric layer.